One of the most commonly used image file compression formats is JPEG, as it can dramatically reduce the saved file size on disk.
FIGURE 1 Here we see the distinct square pattern of compression set too high or the image saved too often.
However, there is a price to pay as this compression is achieved by dividing up the image into 8 × 8 blocks of 64 pixels. If the compression is set too high, or the JPEG file has been saved too often, then a distinct square pattern becomes very noticable. This pattern is unique to JPEG and is well worth remembering as it has all too often ruined quality images.
NOTE: JPEG compression is one of the options when saving images as TIFF and PDF files. This can result in these file formats having the same damage effects.
FIGURE 2 If you think you see posterisation in a gradient (left), but are not sure of its cause, then look through the separate channels for a clearer view (right. Rather than smooth divisions between banding, the JPEG compression causes a jagged block effect in the posterised steps as a result of the squares.
Where excessive JPEG compression has been used on images with gradients, the result can be a banding effect. The same blocks of 8 × 8 pixels are still being formed, but as the JPEG squares reduce the tonal differences between the pixels it can make other squares across the gradient appear more and more similar. When the compression is severe enough the gradient eventually posterises.